1. Field of the Invention
The present invention relates to a solid electrolytic capacitor and a method of manufacturing the same, and more particularly, it relates to a wound type solid electrolytic capacitor and a method of manufacturing the same.
2. Description of the Background Art
With the recent digitization and increase in frequency of electronic equipment and the increase in the temperature for reflow soldering with lead-free solder, a miniature high-volume capacitor excellent in low impedance characteristics in a high-frequency domain and heat resistance is required.
Such requirements for miniaturization, increase in volume and low impedance characteristics in a high-frequency domain can be satisfied by a wound type electrolytic capacitor obtained by storing a capacitor element formed by winding a cathodic foil and an anodic foil through a separator in a metal case and sealing the same with sealing rubber. A solid electrolytic capacitor having a solid electrolytic layer made of a conductive polymer such as polypyrrole or polythiophene exhibiting excellent conductivity is provided.
Such a solid electrolytic capacitor is subjected to reflow soldering with lead-free solder, having a melting point considerably higher than that of conventional lead solder, at a high temperature of 200 to 270° C. If the solid electrolytic capacitor having the electrolyte of the conductive polymer such as polypyrrole or polythiophene is subjected to reflow soldering under such a temperature condition, swelling of the sealing rubber or the metal case or deterioration of various electric characteristics disadvantageously excessively progresses due to cracked gas resulting from deterioration of the conductive polymer.
It has been recognized that the cracked gas results from deterioration of the conductive polymer for the following reason: Thermal decomposition of synthetic cellulose made from natural fiber generally used for the separator starts from a temperature of about 150° C., and deterioration of the conductive polymer acceleratedly progresses due to this decomposition, to generate the cracked gas.
Therefore, employment of aramid fiber which is an organic polymer having a high coefficient of elasticity and excellent heat resistance as a separator is proposed (refer to Japanese Patent Laying-Open Nos. 2002-203750 and 2002-252147, for example).
However, a solid electrolytic capacitor having a separator of aramid fiber is inferior in adhesion between a conductive polymer employed as a solid electrolyte and the separator. Consequently, equivalent series resistance (hereinafter abbreviated as ESR) is disadvantageously increased.
The aramid fiber has high strength, a high coefficient of elasticity and high heat resistance due to properties such as high orientation. However, the surface of the aramid fiber is so inactive that the interface between the aramid fiber and a polyelectrolyte is inferior in strength or adhesion. According to a method of activating the inactive surface of the aramid fiber (refer to Japanese Patent Laying-Open No. 2004-164974, for example), the heat resistance of the aramid fiber is reduced due to chemical treatment. Consequently, the material characteristics of the aramid fiber for serving as the separator of a solid electrolytic capacitor are remarkably reduced.